Emission control legislation requires that the exhaust gases of internal combustion engines are treated prior to discharge from the exhaust tail pipe. Typically this treatment includes a reduction in the level of particulates and also the conversion, via a catalytic converter, of various undesirable chemicals found within the exhaust stream. The composition of the catalytic converter may depend on the fuel system used by the vehicle as different catalysts are optimised to deal with the exhaust gases from diesel and gasoline engines.
The chemical reactions that are undertaken within the catalytic converter have a temperature envelope in which they operate effectively. Below what is commonly referred to as the “light off” temperature, the catalytic converter does not operate effectively, which may result in unacceptable levels of some pollutants remaining within the exhaust stream. It is therefore desirable for the catalytic converter to reach “light off” temperature as soon as possible after the engine is started to mitigate the effects of the cold engine, which is prone to produce a higher of some pollutants than the engine at normal operating temperature.
The temperature of the catalytic converter is raised by the exhaust gases that are incident on it and also as a result of its proximity to other engine components that become hot when the engine is running. As combustion engines become more and more fuel efficient, the time taken for the catalytic converter to reach “light off” temperature may increase.
In order to promote efficient driving, many internal combustion engines are provided with one or more additional providers of boost. These may be turbochargers or superchargers. In twin charged engines, or multi-staged boosted engines, both a turbocharger and a supercharger may be provided.
In a twin charged engine, the supercharger is provided to improve the low-rpm performance of the engine and also to mitigate the time delay between the application of the throttle and the provision of the required boost from the turbocharger.
The present disclosure provides a reduction in catalyst “light off” time for a twin charged internal combustion engine.
According to the present disclosure there is provided a twin charged engine comprising a catalytic converter; a first compressor which, when operated, increases engine load; a second compressor which extracts energy from the exhaust gases to increase the overall engine efficiency; and a controller configured to operate one of at least two modes; wherein a first mode is a standard operating mode in which the system is configured to optimise the efficiency of running of the engine; wherein a second mode is for use under special conditions.
The special conditions may include an engine cold start, the regeneration of a Diesel Particulate Filter (DPF), and the desulphation of a Lean NOx Trap (LNT).
When the twin charged engine is operated in the special mode, the controller may be configured to use the first compressor until the catalyst “light off” temperature is reached.
When the twin charged engine is in the standard operating mode, the controller may be configured to use the second compressor in order to optimise the efficiency of running of the engine.
The twin charged engine may further comprise an exhaust gas recirculation system and wherein the controller may be further configured to activate the exhaust gas circulation system whilst the second compressor is bypassed.
The first compressor may be a supercharger, in particular a hybrid belt/electrically driven supercharger or an electric supercharger or a belt driven supercharger.
The second compressor may be a turbo-charger, in particular a fixed geometry turbocharger or a variable geometry turbocharger which may have a waste gate. The catalytic converter may be mounted on the exit of the turbo-charger.
When the twin charged engine is operated in the special mode, the controller may be configured to cause exhaust gases to bypass the turbo-charger by passing through the waste gate. The waste gate may be configured to provide a bypass for substantially all of the exhaust gases.
Furthermore, according to the present disclosure there is provided a method of starting up a twin charged engine; wherein the twin charged engine comprises a catalytic converter, a turbo-charger with a waste gate and a supercharger; the method comprising the steps of: opening the waste gate of the turbo-charger thus bypassing the turbo-charger; starting the engine and using the supercharger to modulate the air-fuel ratio; and monitoring the temperature of the catalytic converter; once catalyst “light-off” temperature is reached, closing the waste gate and reverting to normal operating sequence of the turbo-charger and the supercharger.
Furthermore, according to the present disclosure there is provided a method of operating a twin charged engine, wherein the twin charged engine comprises at least one exhaust gas filter, a turbo-charger with a waste gate and a supercharger; the method comprising the steps of: opening the waste gate of the turbo-charger thus bypassing the turbo-charger; using the supercharger to optimise the air-fuel ratio; regenerating the filter; monitoring the status of the filter; and once the filter has been regenerated, closing the waste gate and reverting to normal operating sequence of the turbo-charger and supercharger.
The disclosure pertains in general to twin charged engines which are well-known in the art and therefore only those aspects pertinent to the present invention will be described in detail.
There is provided a turbo-charger which is provided with a large bypass, typically termed a waste gate. In normal operation, the turbo-charger recycles the heat of the exhaust gases to drive a turbine which, in turn, causes an increase in the air introduced to the engine thereby improving the air to fuel ratio to increase the overall efficiency of the engine.
The bypass is used during normal operation to reduce the power of the turbo-charger. The bypass is configured to ensure minimum heat loss. The outlet of the turbo-charger is also configured to be as large as practical in order to prevent heat loss into the turbo-charger walls. The turbo-charger may have either fixed or variable geometry.
There is also provided a forced induction system driven by the engine. The forced induction system is typically a supercharger which adds to the engine load when it is running. a result, the running of the supercharger increases the energy of the exhaust. The supercharger may be a hybrid belt electrically driven supercharger or any other device capable of providing forced induction such as an electric supercharger or a belt driven supercharger. The can be used to fill in the response characteristics at low engine speeds to compensate for any deficiency from the turbo-charger. This also enables the waste gate to be at least partially under light load conditions in order to reduce the exhaust back pressure and thereby improve the fuel economy. The supercharger can also be used for transient response in order to eliminate turbo-charger lag. The presence of a supercharger may influence the choice of turbo-charger as, in some cases, the presence of a supercharger allows a fixed geometry turbo-charger to be used place of a variable geometry turbo-charger.
There is also provided a catalytic converter which includes one or more catalyst designed to reduce the levels of certain pollutants in the exhaust gas stream. The catalyst or catalysts typically have an envelope of operating temperatures. At the lower limit of this envelope is the so-called “light off” temperature. Until the catalyst is raised to this threshold temperature it does not operate effectively and therefore the levels of certain pollutants within the exhaust gas stream may exceed permitted levels. It is therefore important that the temperature of the catalyst is raised rapidly on engine start up in order to ensure that effective catalysis of the exhaust stream commences as soon as possible after the starting of the engine.
The catalytic converter is mounted on the exit of the turbo-charger in order to maximise thermal transfer from the turbo-charger to the catalytic converter during normal operation.
There is also provided an exhaust gas recirculation system (EGR) which has variable flow depending on the engine condition, i.e. the percentage of the exhaust gases that are recirculated can be changed according to the requirements of the engine. If the EGR flow is high, i.e. a large percentage of the gases are recirculated, this retains heat within the system and further contributes to the rapid heating of the catalytic converter to “light off” temperature. High EGR flow is compatible with the use of the supercharger and bypass of the turbo-charger. As such, the use of the supercharger or other forced induction system can be seen as an enabler for high EGR flow resulting in rapid catalyst “light off” with little heat lost to the turbo-charger because it is bypassed.
Control of these system elements is initiated in the ECU (engine control unit). The controller has at least two modes in which the system can be operated. The first mode is a normal operating mode in which the efficiency of operation is optimised. This includes the preferential use of the turbo-charger to harvest energy that might otherwise be wasted from the exhaust gases. The second, or special, mode reverses this logic and bypasses the turbo-charger, providing boost instead from the supercharger. This mode is appropriate for short term circumstances only, such as the start up from cold of the engine and also for regenerating a DPF or the desulphation of an LNT.
When the engine is started from cold, the ECU sends a signal to open the waste gate to its maximum extent so that the turbo-charger is substantially completely bypassed, or, where the size of the waste gate does not permit a total bypass, the extent of the bypass is maximised for the components of the system as presented. This configuration maximises the volume of hot exhaust gases that can flow directly onto the catalyst, thus heating the catalyst as rapidly as possible towards “light off” temperature.
If boost is required whilst the turbo-charger is bypassed, this is provided using the supercharger.
Opening the waste gate prior to catalyst “light off” has notable advantages. Because this configuration bypasses the turbo-charger, it minimises the boost provided by the turbo-charger during the start up of the engine. When the load is light, the boost level required may be minimal or zero. During this phase an exhaust gas recirculation system can be used to maintain the heat in the system and contribute to the efficient raising of the temperature of the catalyst. If the load on the engine increases prior to catalyst “light off” additional boost can be provided by running the supercharger to increase the air to fuel ratio and thereby provide the requisite combustion control.
The bypass of the turbo-charger is provided via a waste gate is effectively a low pressure path for the exhaust gases and therefore the exhaust gases will travel directly to the catalytic converter so that no heat will be lost from the system within the turbo-charger, as this is effectively bypassed by the use of the waste gate.
The absence of the turbo-charger from the energy flow of the system means that there is no reduction in the exhaust enthalpy as would usually be caused by the running of the turbo-charger. This effect is compounded by the use of the supercharger to provide boost during start up which increases the engine load and therefore the exhaust energy level. As a result the time taken for the catalytic converter to reach “light off” temperature is reduced.
The vehicle ECU is configured to receive data from a plurality of sensors throughout the vehicle, each sensor being configured to provide data about one aspect of the status of one location within the vehicle. Amongst this plurality of sensors will be a number of temperature sensors that provide data to the ECU pertaining to the temperature in various parts of the vehicle including the temperature of the catalyst in the catalytic converter. The capacity of the DPF is also monitored and data is provided to the ECU in order to schedule the regeneration of the DPF. The sensors typically measure the temperature and back pressure through the DPF as the back pressure will increase with the accumulation of particulate matter. Regeneration can only take place when the temperature exceeds a predetermined threshold value for a specified time. If the vehicle undertakes lengthy high speed drives for example on motorways, then DPF regeneration can take place during normal driving. However, if the vehicle is primarily used for town driving then it may be difficult to achieve the conditions for a standard regeneration.